This invention relates to a resolution heightening circuit, particularly, relates to a resolution heightening circuit which is used in an electrophotographic printer to apparently heighten resolution of the electrophotographic printer.
An electrophotographic printer has a printing unit including a driving circuit. In general, performance of the printing unit decides resolution of the electrophotographic printer. That is, resolution of the printing unit is equal to the resolution of the electrophotographic printer.
The performance of the printing unit is represented by the maximum number of lateral printing lines that can be printed on a paper having a predetermined size and the maximum number of printing dots that can be printed on each of the lateral printing lines having a predetermined length. In other wards, the performance of the printing unit is represented by the maximum numbers of dots, which the printing unit can print along longitudinal and lateral directions, per inch.
If the performance of the printing unit is elevated, the resolution of the electrophotographic printer will be heightened. Such a technique is disclosed in each of Japanese Unexamined Patent Publication No. 8-185021, Japanese Unexamined Patent Publication No. 10-171219, and Japanese patent No. 2673958.
However, elevation of the printing unit""s performance is expensive because it needs high precision in control for the driving circuit.
So some proposals are made to apparently heighten resolution of the electrophotographic printer without improvement of the driving circuit.
For example, Japanese Unexamined Patent Publication No. 4-336859 discloses a high density image forming method. In the high density image forming method, a laser beam is irradiated and scanned on a surface of a photosensitive element of a printing unit to form beam dots each of which partially overlaps adjacent beam dots. Strength of the laser beam is changed to form a latent image pattern on the surface of the photosensitive element. Namely, the strength of the laser beam is adjusted so that some beam dots individually form latent image dots and the other beam dots forms latent image dots at overlapped areas of the beam dots. Thus, the high density image forming method obtains seeming heightened resolution.
However, the high density image forming method provides disappointing result because it doesn""t adopt a pulse width modulation technique and can not represent gradation. In addition, the high density image forming method has a fault that it is difficult to control the strength of the laser beam.
Another method for heightening resolution of the electrophotographic printer is disclosed in Japanese Unexamined Patent Publication No. 5-500433. The method apparently doubles resolution of a printing unit by using a reference template and image data signal, which corresponds to double the resolution of the printing unit.
However, the method brings the same result from certain different patterns of the reference template because the reference template has a size of two lateral lines by four dots (i.e. 8 pixels). That is, the method doesn""t always apparently double the resolution of the printing unit.
Moreover, the method needs a large memory to memorize a large number of modulation information which correspond to all patterns of the reference template.
It is therefore an object of this invention to provide a resolution heightening circuit that can apparently heighten resolution of a printing unit.
It is another object of this invention to provide a resolution heightening circuit that drives a printing unit with simple control.
It is still another object of this invention to provide a resolution heightening circuit that makes a printing unit faithfully print in response to input data.
It is further still another object of this invention to provide a resolution heightening circuit that is a small-scale circuit.
Other object of this invention will become clear as the description proceeds.
On describing the gist of this invention, it is possible to understand that a resolution heightening circuit is connected between a printing unit, which has a first resolution, and a host printer control circuit, which produces printing data for a second resolution grater than the first resolution. The resolution heightening circuit produces modulated driving pulses on the basis of the printing data and supplies the modulated driving pulses to the printing unit to apparently heighten resolution of the printing unit.
According to the gist of this invention, the resolution heightening circuit comprises a modulation information generating circuit that is connected to the host printer control circuit and that generates a pulse width modulation information signal on the basis of the printing data to increase the number of dots for a line at the printing unit. A modulation circuit is connected to the modulation information generating circuit and the printing unit and produces the modulated driving pulses in response to the pulse width modulation information signal to supply the modulated driving pulses to the printing unit.
In the resolution heightening circuit, the modulation information generating circuit comprises a line data memorizing circuit that is connected to the host printer control circuit to memory the printing data. A template pattern producing circuit is connected to the line data memorizing circuit and reads out the printing data from the line data memorizing circuit to produce template patterns of four lines by three dots on the basis of the printing data. A modulation information forming circuit is connected to the template pattern producing circuit and the modulation circuit and forms the pulse width modulation information signal from the template patterns to supply the pulse width modulation information signal to the modulation circuit.
Moreover, the modulation information forming circuit comprises first, second, and third processing circuits that are connected to the template pattern producing circuit and processes left, middle, and right columns of each of the template patterns to produce first, second, and third processed value signals, respectively. A pulse width deciding circuit is connected to the second processing circuit and decides width of the driving pulses on the basis of the second processed value signal to produce a width signal representative of the width. A pulse position deciding circuit is connected to the first and the second processing circuits and decides position of the driving pulses on the basis of the first and the third processed value signal to produce a position signal representative of the position. A pulse width modulation information producing circuit is connected to both the pulse width deciding circuit and the pulse position deciding circuit and produces the pulse width modulation information signal in response to the width signal and the position signal.
In addition, the pulse width deciding circuit comprises a plurality of pulse width setting registers that registers pulse width setting data sets different from one another, respectively. A first selector is connected to the pulse width setting registers and the second processing circuit and selects one from the pule width setting registers in response to the second processed value signal to supply the pulse width setting data set registered in selected one of the pulse width setting registers to the pulse width modulation information producing circuit as the width signal. The pulse position deciding circuit comprises a subtracting circuit that is connected to the first and the third processing circuits to subtract a value represented by the third processed value signal form a value represented by the first processed value signal to produce a subtracted result signal. A plurality of pulse position setting registers register pulse position setting data sets different from one another, respectively. A second selector is connected to the subtracting circuit and the pulse position setting registers and selects one of the pulse position setting register in response to the subtracted result signal to supply the pulse position setting data set registered in selected one of said pulse position setting registers to the pulse width modulation information producing circuit as the position signal.